The present invention relates generally to power steering systems for vehicles, and more particularly to an electrically powered steering system intended for large vehicles.
Virtually all present power steering systems comprise implementation means whose fundamental output is force based. By way of example, present art power steering systems generally comprise a common open-center valve that delivers differential pressure to a double-acting power cylinder as a function of torque applied to a steering wheel. This is accomplished via the steering wheel progressively closing off return orifices comprised within the open-center valve. Another example is an electric power steering system (hereinafter “EPS system”) wherein a servo-motor delivers torque as a function of current applied to it by a controller. An EPS system of particular interest herein is described in U.S. Pat. No. 6,152,254, entitled “Feedback and Servo Control for Electric Power Steering System with Hydraulic Transmission”, issued Nov. 28, 2000 to Edward H. Phillips, wherein differential pressure is directly delivered to a double-acting power cylinder from a servo-motor driven reversible fluid pump. Because of continued reference below to the '254 patent, the whole of that patent is expressly incorporated by reference herein.
While the EPS system described in the incorporated '254 patent has optimum performance characteristics, it like all EPS systems is limited in utilization to relatively small vehicles because of limited available electrical power. All vehicle manufacturers limit electrical current availability for EPS systems to a value that can be supplied directly from an alternator. A limiting value of perhaps 70 Amperes from a 12 Volt electrical system is typical. At a lower limiting voltage value of 10 Volts and an overall EPS system efficiency of perhaps 60% this results in a net maximum power delivery from the steering gear of only 420 Watts. This low value stands in stark contrast to known future power steering system requirements ranging as high as 3,500 Watts.
Various so-called “closed-center” power steering systems have been proposed as a solution to this problem. Such closed-center power steering systems utilize an accumulator to store power steering fluid at relatively high pressure. Some form of closed-center valving is then used to meter a flow of pressurized fluid to one end of a double-acting power cylinder while concomitantly permitting a similar return flow of low pressure fluid from the other end to a reservoir. Pressurized fluid is supplied to the accumulator from the reservoir by a relatively small displacement pump driven by a simple (e.g., non-servo) motor controlled by a pressure activated switch.
To date however, none of the proposed closed-center power steering systems has provided acceptable on-center steering “feel” and they have not gained acceptance in the industry. It is believed herein that the primary problem with the closed-center power steering systems proposed to date is that their fundamental output is fluid flow or rate-based rather than force-based as is described above with reference to currently accepted power steering systems. The rate-based closed-center systems provide nominally linear control of system velocity with inherent discontinuities in system acceleration while the force-based systems directly provide linear control of system acceleration. It is believed herein that these discontinuities in system acceleration are the root cause of the unacceptable on-center steering feel in the closed-center power steering systems.
Therefore, it would be highly advantageous to provide an accumulator and closed-center valve enabled power steering system that has the acceptable on-center steering “feel” provided by a force-based power steering system.